A species of the Astraeidae (Star-corals) of the reef-forming section, _Plesiastraea st.vincenti_, is found in the Kalimnan of Hallett's Cove, South Australia. _HYDROZOA._ The few animals of this group met with in fossil faunas are represented by the living _Millepora_ (abundant as a coral reef organism), _Hydractinia_ (parasitic on shells, etc.), and _Sertularia_ (Sea-firs). =Milleporids and Stylasterids.--= Although so abundant at the present time, the genus _Millepora_ does not date back beyond the Pleistocene. The Eocene genus _Axopora_ is supposed to belong here, but is not Australian. Of the Stylasterids one example is seen in _Deontopora_, represented by the branchlets of _D. mooraboolensis_, from the Janjukian limestone of the Moorabool Valley, near Geelong. =Hydractinia.--= _Hydractinia_ dates from the Upper Cretaceous rocks in England, and in Australia its encrusting polypidom is found attached to shells in the polyzoal limestone of Mount Gambier (Miocene). Stromatoporoids. An important group of reef-builders in Palaeozoic times was the organism known as _Stromatopora_, and its allies. The structures of these hydroid polyps resemble successional and repetitional stages of a form like _Hydractinia_. As in that genus it always commenced to grow upon a base of attachment such as a shell, increasing by successive layers, until the organic colony often reached an enormous size, and formed great mounds and reefs (see _antea_, Fig. 32). The stromatoporoid structure was formed by a layer of polyp cells separated by vertical partitions, upon which layer after layer was added until a great vertical thickness was attained. This limestone-making group first appeared in the Silurian, and probably reached its maximum development in Middle Devonian times, when it almost disappeared, except to be represented in Carbopermian strata by a few diminutive forms. [Illustration: =Fig. 72.--STROMATOPOROIDEA and CLADOPHORA.= A--Actinostroma clathratum, Nich. Devonian. Rough Range, W.A. B--Actinostroma clathratum, Nich. Devonian. Rough Range, W.A. Vertical section. (_After G. J. Hinde_). C--Callograptus sp. Up. Ordovician. San Remo, Vict. (_After T. S. Hall_). D--Ptilograptus sp. Up. Ordovician. San Remo, Vict. (_After T. S. Hall_). E--Dictyonema pulchellum, T. S. Hall. L. Ordov. Lancefield, Vict. F--Dictyonema macgillivrayi, T. S. Hall. L. Ordov. Lancefield, Vict. ] =Silurian Stromatoporoids.--= In the Silurian limestones of Victoria (Lilydale, Waratah Bay, Walhalla and Loyola), and New South Wales (near Yass), Stromatoporoids belonging to the genera _Clathrodictyon_ (probably _C. regulare_), _Stromatopora_ and _Idiostroma_ occur. _Stromatoporella_ has been recorded from the Silurian rocks of the Jenolan Caves, New South Wales. =Devonian Stromatoporids.--=